The fear of hypoglycemia and hypoglycemia unawareness limits the ability of patients with diabetes from achieving the level of glucose control known to reduce the risk of developing microvascular complications. The long term goal of this project is to define the pathogenesis of hypoglycemia-associated autonomic failure (HAAF) with the aim of helping patients better manage their diabetes. The working hypotheses for this project are: 1) recurrent hypoglycemia, like that seen in subjects with type 1 diabetes and iatrogenic hypoglycemia, will lead to an upregulation of glucose availability for the brain (by increased transport and/or increased glycogen storage), thereby facilitating maintenance of cerebral metabolism during subsequent episodes of hypoglycemia, and 2) recurrent hypoglycemia will lead to an increased GABA tone in hypothalamus, thereby suppressing glucose counter regulation during subsequent episodes of hypoglycemia. To address these hypotheses we will examine cerebral metabolism in subjects with type 1 diabetes and hypoglycemia unawareness and in healthy volunteers preconditioned with euglycemia vs. recurrent hypoglycemia to induce HAAF, thereby permitting us to understand the clinical syndrome as well as isolate the effects of recurrent hypoglycemia from those induced by diabetes. We will use magnetic resonance spectroscopy at high and ultra-high fields to examine hypothalamic glucose and GABA metabolism, as well as cerebral glycogen metabolism. Aim 1: To determine the contribution of increased glucose availability in the brain to HAAF Specific Aim 1a: To measure kinetic parameters for glucose transport and metabolism in the hypothalamus of subjects with type 1 diabetes and hypoglycemia unawareness and in healthy volunteers preconditioned with euglycemia vs. recurrent hypoglycemia to induce HAAF. Specific Aim 1b: To measure brain glycogen turnover and content by 13C MRS in healthy volunteers preconditioned with euglycemia vs. recurrent hypoglycemia to induce HAAF. Aim 2: To determine the contribution of increased GABA tone in the hypothalamus to HAAF Specific Aim 2: To measure euglycemic and hypoglycemic GABA levels in the hypothalamus of subjects with type 1 diabetes and HAAF and in healthy volunteers preconditioned with euglycemia vs. recurrent hypoglycemia to induce HAAF.